In optical systems, it is often desirable to use a bandpass filter. One traditional form of optical bandpass filter involves a substrate with a coating containing multiple layers of optical materials. However, the number of layers needed in the coating can often exceed 100-150 layers, resulting in a very high cost. Moreover, the effective bandwidth and the center wavelength are essentially fixed during manufacture, and can only be tuned by a very small amount (always shorter and narrower), in particular by tilting the filter relative to an incident beam. However, at higher angles of incidence, the amplitude transmission deteriorates. Also, due to the transmissive nature of the filter, it can be difficult to design a coating that provides a passband for certain wavelengths ranges. For example, the substrate and/or coating materials may tend to absorb radiation in the ultra violet range.
Another consideration is that optical alignment problems can result from deviation imparted to the beam as the beam passes through the substrate. Still another consideration is that the relatively large number of coating layers can induce substrate curvature, due to tensile and/or compressive stresses stacking up in the coating. This can cause wavefront distortion and/or beam deviation, resulting in optical misalignment problems in sensitive optical systems.
According to a different approach, a beam is routed successively through two separate edge filters, one of which passes longer wavelengths, and the other of which passes shorter wavelengths. Each of these filters has a gradient-thickness coating provided on a plane-parallel substrate. In other words, each has a substrate of uniform thickness, with a coating that progressively increases or decreases in thickness along the substrate. The coating may be a multi-layer coating, where each layer progressively increases or decreases in thickness along the substrate. The two filters can be moved in a lengthwise direction with respect to each other, or in other words approximately perpendicular to the direction of travel of radiation. As a result of this relative movement, the width of the passband increases or decreases. However, the manufacture of gradient-thickness coated filters is complex and expensive. Also, due to the transmissive nature of the filters, absorption and beam deviation can be problems.
The types of optical bandpass filters mentioned above have been generally adequate for their intended purposes but, as noted in the foregoing discussion, they have not been satisfactory in all respects.